Method of and apparatus for the automatic refocusing of x-ray spectrometers

ABSTRACT

In fully focusing X-ray spectrometers, in order to correct the measuring error caused by a change of the Bragg-angle due to the excursions of the electron beam that scans the sample, the diffracting crystal is rotated synchronously to cancel said change. The rotation of the crystal is effected by an electromotor energized by a voltage derived from a comparison of a DC sawtooth voltage (the electron beam-deflecting voltage) with a DC voltage varied by said electromotor.

United States Patent Inventor App]. No. Filed Patented Priority METHOD OF AND APPARATUS FOR THE AUTOMATIC REFOCUSING OF X-RA Assistant Examiner-A. L. Birch Attorney-Edwin E. Greigg ABSTRACT: In fully focusing X-ray spectrometers, in order to correct the measuring error caused by a change of the ig i g g m Bragg-angle due to the excursions of the electron beam that "wing scans the sample, the difiracting crystal is rotated U.S. Cl 250/495, synchronously to cancel said change. The rotation of the 250/5l.5 crystal is effected by an electromotor energized by a voltage Int. Cl ..G0ln 23/22 derived from a comparison of a DC sawtooth voltage (the Field of Search 250/495 electron beam-deflecting voltage) with a DC voltage varied by (8), 51.5 said electromotor.

f L -l PATENTEDnm 12 ISTI SHEET 2 OF 2 BACKGROUND OF THE INVENTION In the process of electron beam microanalysis, fully focusing X-ray spectrometers are used for the resolution of the characteristic X-ray radiation. The known apparatus of this type, while ensuring a very high efficiency in the detection of x-rays, have the disadvantage in that only very limited areas of the sample meet the conditions for focusing. In the plane of the focusing circle, the so-called Rowland circle, the range in which the conditions for focusing couldstill be met without substantial error is, depending upon the structure of the X-ray spectrometer, about 200-500 11.. In all directions normal to said plane, the tolerable deviation from the exact focusing point is merely 20-100 1. and, in X-ray spectrometers of highest resolution, this distance is even smaller. A deviation of the electron beam in excess of these values causes a very significant drop in the X-ray count and, therefore, simulates a nonexistent decrease in the concentration of the selected element on the sample.

In the electron beam microanalysis, however, an excursion of the electron beam over larger ranges (even beyond the Rowland circle), is extremely important for performing the scanning of surfaces. More recently, in order to satisfy both requirements-that is, on the one hand, a strict compliance with the Rowland circle conditions and, on the other hand, a line-byline scanning of the sample with the electron beam-it was the sample and not the electron beam that was caused to move. In more recent electron beam microsondes, electron beam is deflected in the plane of the Rowland circle, while the sample is moved perpendicular thereto (semielectronic scanning).

These mechanical and electromechanical scanning processes, however, have certain inherent basic difficulties. The construction of an accurately operating mechanical scanning device is very difficult and its use has to be limited to small samples due to the large carrying forces necessary for large samples. Rotary motions of the sample or heating experiments may be effected only with the greatest difficulties, if at all. Further, the semielectronic scanning process has to be limited to the use of spectrometers diametrically secured to a microsonde, since only then may the planes (one per spectrometer) of the Rowland circle be caused to coincide exactly. Should it be desired to use a third spectrometer arranged at 90with respect to the two other ones, the aforenoted process for this third spectrometer may not be used. The same applies to other arrangements.

SUMMARY OF THE INVENTION The invention relates to a method of and apparatus for the automatic refocusing of X-ray spectrometers in which the crystal axis, the detector and the X-ray source (formed by the scanning electron beam of a microsonde at the points of impact on the surface of the sample), are disposed on a Rowland circle. According to the invention the electron beam, controlled by deflecting voltages, scans surface of the sample lineby-line; and the x-rays, generated by the electron beam, are reflected from the crystal into the detector and, further, the crystal axis is rotated in such a manner that the angle formed by the line connecting the impact point of the electron beam with the crystal axis and the line normal to the latter is constant for all positions of the electron beam with respect to the sample. In the refocusing of the spectrometer by the travelling electron beam, the effect is geometrical which may be compensated by resetting the angular position of the goniometer (the mechanical device in the spectrometer which carries and moves the defracting crystal and the X-ray detector). The resetting is automatically and independently performed for he individual spectrometers secured to the microsonde.

The invention will be better understood, and objects, as well as advantages, will become apparent from the ensuing detailed specification of a preferred, although exemplary, embodiment taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective schematic view of parts of the preferred apparatus showing the principle of operation; and

FIG. 2 is a schematic diagram showing control means incorporated in the preferred embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now to FIG. 1, a sample 1 is scanned line-by-line 0 with an electron beam 2. For this purpose the electron beam 2 may consecutively travel along perpendicular paths. The focusing point F of the electron beam 2 on the sample I forms an X-ray source which travels with the scanning beam and which emits X-rays whose wavelength is measured by means of an X-ray spectrometer. The spectrometer used for this purpose is of the fully focusing type for which the Bragg condition applies and in which the X-ray source (the focusing point F), the detector 3 and the crystal 4 lie on the Rowland circle R, the plane of which is normal to the surface of the sample 1. The crystal 4 is arranged in such a manner on a goniometer head 5 that the crystal axis A and two edge faces of crystal 4 are disposed normal to the plane of the Rowland circle R and intersect or almost intersect the area within the Rowland circle. The other two edge faces of the crystal 4 extend in a plane parallel to the plane of the Rowland circle. The crystal axis A thus extends practically parallel to the surface of the sample I so that during a scanning of the surface of sample I in a direction parallelto the crystal axis A, the angle 8 which is formed, on the one hand, by the line B connecting the focusing point F with the crystal axis A and, on the other hand, the line N normal to A, remains constant. The angle Y complementing the angle ,3 to is the Bragg-angle which, in the aforementioned scanning process, also remains constant.

The Bragg-angle Y, however, changes during a scanning of the sample I in a direction normal to the scanning just described. The direction of such scanning coincides with one chord of the Rowland circle R so that the angle a which is formed between the direction of motion and the line connecting the travelling focusing point F with the crystal axis A varies. It is a result of this angular variation that, for example, in a constant position of crystal 4--i.e., the Bragg-angle is set to a characteristic X-ray line-the scanning of the sample 1 yields erroneous data (concentration of a certain element in the entire sample I). Approaching the marginal regions of the sample 1, the intensity (count rate) of the X-ray radiation decreases since, due to the aforenoted change in angle, an intensity is measured which is outside the range of the maximum of a line which, in an ideal case, is a bell-shaped ascending curve. Thus, the line intensity is measured as a function of the Bragg-angle. Relating this to the entire sample I, a concentration of a certain substance would be simulated which lies under the real value.

With the novel process, according to the invention, this erroneous measuring is eliminated by rotating the crystal axis A and thus the crystal 4 synchronously with the change in angle occurring during the scanning in a chordal direction. This synchronous movement proceeds in the direction of cancelling the change in angle. In the case of linear spectrometers, the rotational movements of the crystal axis A in the direction of the line connecting the crystal axis A with the focusing point F. This translational motion, however, is not essential to the solution of the problem. A synchronous rotational movement of the crystal 4 alone is sufficient. This lastnamed possibility is illustrated in FIG. 1. As seen, the crystal 4 is secured to a goniometer head 5 to which there is affixed a worm wheel 6 meshing with worm 7. Shah 8, to which worm 7 is secured, is driven through a clutch 9 by an electromotor 10. The translational motion of worm 7 resulting from a rotation by shafl 8 effected by motor 10 is transmitted through the worm wheel 6 as a rotary motion directly to the goniometer head 5. The aforedescribed angular displacements of the crystal 4 to compensate for the defocusing of the spectrometer caused by the scanning motion of the electron beam 2 result in such small differences between the new and the normal angular positions of the crystal 4 that the detector 3 need not follow in synchronism the rotary motion of the crystal 4. Thus, unless an extremely narrow slit is used between the crystal 4 and the detector 3, the latter may remain stationary and still pick up without appreciable loss the rays reflected by the crystal 4 in its angularly adjusted position.

The synchronization of the deflection of the electron beam 2 with the rotation of the crystal 4 is effected by an apparatus diagrammatically illustrated in FIG. 2. A sawtooth generator 11, which delivers a voltage U, for the deflection of the electron beam 2 in the direction of a chord of the Rowland circle R, also delivers the same varying DC voltage U, to one input terminal of differential amplifier 12. To the other input terminal of differential amplifier 2 there is applied a DC voltage U taken from a potentiometer 13 by means of a slider 14. The potentiometer 13 is connected to a DC voltage source V. The two voltages U, and U, are adjusted with respect to one another in such a manner that for extreme or peak values (maximum or minimum) of the voltage U, the two voltages U, and U, are in balance at the input terminals of the differential amplifier 12. Consequently, in such cases the voltage at the output terminal G of the differential amplifier 12 is zero. Turning once again to FIG. 1, the maxima and minima of the sawtooth voltage U, correspond to the maximum excursions of electron beam 2 on sample 1. This then means that for each peak value of the sawtooth voltage U,, the angular deviation from the Bragg-angle Y is at its maximum if the angular position of crystal 4 is fixed.

in order to correct the aforenoted angular deviation, the crystal 4, while maintaining a constant position with respect to the line connecting the travelling focusing point F with the crystal axis A, has to be rotated clockwise or counterclockwise into extreme positions. It is to be noted that in linear spectrometers a translational movement in the direction of said connecting line is superimposed on the rotational movements. Such translational movement, however, does not give rise to additional errors.

lf now the voltage U, changes at the output of the sawtooth generator 11, the measuring bridge (not shown) in difierential amplifier 12 becomes unbalanced so that the latter supplies a voltage AU (proportional to the voltage difference U,U at the input terminals of amplifier 12) to the electromotor 10. As a result, the electromotor 10 rotates in a direction depending upon the voltage polarity MU and moves the slider 14 of the potentiometer 13 as well as the goniometer head 5 through the worm 7 of the shaft 8. The electromotor 10 will stop rotating when the potentiometer l3 delivers a voltage U, which is equal to the voltage U, at the output of the sawtooth generator 11 resulting in zero voltage at amplifier output G (AU=O).

For an optimal adjustment of the apparatus the following steps should be taken:

The shaft 8 is disconnected from the electromotor 10 by opening the clutch 9 and the maximum intensity of the selected X-ray line in one extreme position of the electron beam 2 is manually adjusted at the goniometer. Further, the slider 14 of the potentiometer 13 is set to zero. Thereafter, the electromotor l0 and the goniometer head 5 are connected and the electron beam 2 is brought into its other extreme position by changing the setting at the sawtooth generator 11 accordingly. At the same time, the slider 14 rotates into a determined position between the two terminals of the potentiometer 13. Thus, the slider 14 and the crystal 4 on the goniometer head 5 have rotated through a determined angle. The intensity of the X-ray line measured in this position in most cases does not correspond to the maximum of the X-ray line. in order to obtain such a maximum, and thus to achieve that in both extreme positions (maximum deflections) and in the intermediate positions of the electron beam 2 always the maximum of the X-ray line is measured with the detector D, the required rotation of the goniometer head is set by means of an appropriate adjustment of the variable voltage source V. Such an arrangement may be made purely empirically or may be based on tabulated data depending upon the Bragg-angle Y, the

magnitude of the electron beam deflection and the dimensions of the diffracting crystal 4.

In further embodiments of the invention, the sawtooth voltage U, required for the control of the deflection of electron beam 2 may be used as a second, control voltage generated in the same sawtooth generator 11 and synchronized with the deflecting voltage U,. Further, the sawtooth voltage U for the differential amplifier 12 may be generated independent of the generator that produces the deflecting voltage for the electron beam 2.

What is claimed is:

1. In an X-ray spectrometer of the type having an X-ray source, a diffracting crystal and a detector all lying on a Rowland circle, said X-ray source being formed by an electron beam at the point of impact on the surface of a sample being scanned by said beam, the method of automatically refocusing said spectrometer comprising the following steps:

A. scanning said surface by said electron beam controlled by a deflecting voltage,

B. directing the x-rays from said X-ray source to said crystal for reflection into said detector, and

C. rotating said crystal about its axis normal to said Rowland circle in such a manner that the angle formed between the line connecting said point of impact of the scanning electron beam with said axis and a line normal to said axis is constant for all positions of said electron beam with respect to said sample.

2. A method as defined in claim 1, wherein said electron beam is deflected in a chordal direction with respect to said Rowland circle by a sawtooth voltage, said crystal is rotated about said axis by means energized by a voltage which is a function of said sawtooth voltage.

3. A method as defined in claim 2, wherein said voltage for energizing said means is derived from a comparison of said sawtooth voltage with a DC voltage which is a function of the angular position of said crystal.

4. A method as defined in claim 3, wherein said DC voltage is set in such a manner that said voltage for energizing said means is zero at instants when said sawtooth voltage is at a peak value.

5. In an X-ray spectrometer of the type having an X-ray source, a diffracting crystal and a detector, all lying in a Rowland circle, said X-ray source being formed by an electron beam at the point of impact on the surface of a sample being scanned by said electron beam, the improvement for automatically refocusing said spectrometer, comprising,

A. sawtooth voltage generator means for producing a first, sawtooth voltage utilized to effect a deflection of said electron beam in the direction of a chord of said Rowland circle,

B. a potentiometer supplying a second, DC voltage,

C. an electric motor adapted to rotate said crystal to adjust its angular position, said electric motor being rotatable by an energizing voltage, and

D. 1. differential amplifier having 1. a first input terminal receiving said sawtooth voltage,

2. a second input terminal receiving said second DC voltage,

3. an output terminal supplying said energizing voltage to said electric motor, said energizing voltage being a function of the difference between said first and second voltages. V

6. The improvement as defined in claim 5 wherein said potentiometer includes a slider, said electric motor is connected to said slider to vary said second DC voltage as a function of the angular position thereof.

7. The improvement as defined in claim 5, including a shaft adapted to be rotated by said electric motor, a worm arranged coaxially with said shaft and rotating therewith, a worm wheel meshing with said worm and rotatable thereby, a goniometer head carrying said crystal and fixedly secured to said worm wheel.

8. The improvement as defined in claim 7, wherein said shaft is connected to said electric motor by clutch means.

12. The improvement as defined in claim 5, wherein said sawtooth voltage and the voltage required for the deflection of said electron beam are produced in one sawtooth generator.

13. The improvement as defined in claim 5, wherein said sawtooth voltage is produced by voltage generator means independently of the voltage required for the deflection of said electron beam. 

1. In an X-ray spectrometer of the type having an X-ray source, a diffracting crystal and a detector all lying on a Rowland circle, said X-ray source being formed by an electron beam at the point of impact on the surface of a sample being scanned by said beam, the method of automatically refocusing said spectrometer comprising the following steps: A. scanning said surface by said electron beam controlled by a deflecting voltage, B. directing the x-rays from said X-ray source to said crystal for reflection into said detector, and C. rotating said crystal about its axis normal to said Rowland circle in such a manner that the angle formed between the line connecting said point of impact of the scanning electron beam with said axis and a line normal to said axis is constant for all positions of said electron beam with respect to said sample.
 2. A method as defined in claim 1, wherein said electron beam is deflected in a chordal direction with respect to said Rowland circle by a sawtooth voltage, said crystal is rotated about said axis by means energized by a voltage which is a function of said sawtooth voltage.
 2. a second input terminal receiving said second DC voltage,
 3. an output terminal supplying said energizing voltage to said electric motor, said energizing voltage being a function of the difference between said first and second voltages.
 3. A method as defined in claim 2, wherein said voltage for energizing said means is derived from a comparison of said sawtooth voltage with a DC voltage which is a function of the angular position of said crystal.
 4. A method as defined in claim 3, wherein said DC voltage is set in such a manner that saiD voltage for energizing said means is zero at instants when said sawtooth voltage is at a peak value.
 5. In an X-ray spectrometer of the type having an X-ray source, a diffracting crystal and a detector, all lying in a Rowland circle, said X-ray source being formed by an electron beam at the point of impact on the surface of a sample being scanned by said electron beam, the improvement for automatically refocusing said spectrometer, comprising, A. sawtooth voltage generator means for producing a first, sawtooth voltage utilized to effect a deflection of said electron beam in the direction of a chord of said Rowland circle, B. a potentiometer supplying a second, DC voltage, C. an electric motor adapted to rotate said crystal to adjust its angular position, said electric motor being rotatable by an energizing voltage, and D.
 1. differential amplifier having
 6. The improvement as defined in claim 5 wherein said potentiometer includes a slider, said electric motor is connected to said slider to vary said second DC voltage as a function of the angular position thereof.
 7. The improvement as defined in claim 5, including a shaft adapted to be rotated by said electric motor, a worm arranged coaxially with said shaft and rotating therewith, a worm wheel meshing with said worm and rotatable thereby, a goniometer head carrying said crystal and fixedly secured to said worm wheel.
 8. The improvement as defined in claim 7, wherein said shaft is connected to said electric motor by clutch means.
 9. The improvement as defined in claim 5, including a DC voltage source, the voltage output of which is applied to the terminals of said potentiometer.
 10. The improvement as defined in claim 9, wherein said voltage out put is variable.
 11. The improvement as defined in claim 5, wherein said sawtooth voltage is the voltage required for the deflection of said electron beam.
 12. The improvement as defined in claim 5, wherein said sawtooth voltage and the voltage required for the deflection of said electron beam are produced in one sawtooth generator.
 13. The improvement as defined in claim 5, wherein said sawtooth voltage is produced by voltage generator means independently of the voltage required for the deflection of said electron beam. 